Gonadotropins (luteinizing hormone [LH]) and follicle stimulating hormone (FSH) regulate the biosynthesis of steroid hormones in gonadal cells. This chapter describes the important steps in steroid hormone biosynthesis in the cells of the ovary and testis, with particular emphasis on the sites that are responsive to LH action. LH or its analog, human chorionic gonadotropin (hCG), can increase steroid production acutely as well as by trophic stimulation. Both the acute and the trophic effects of LH/hCG appear to be mediated mostly, if not entirely, by increases in cyclic AMP. The trophic effects involve an increase in steroid biosynthetic enzymes, as well as an increase in membrane receptors for lipoprotein particles which can deliver the substrate, cholesterol, to the cell to be further metabolized to active steroid hormones. The most likely effect of acute LH stimulation is to increase the rate of association of the substrate, cholesterol, with the mitochondrial side-chain cleavage enzyme, as has been reported for ACTH in adrenal cells. 1 , 2 However, the exact mechanism(s) by which acute or trophic stimulation occurs remains to be elucidated.